Fuel cells produce electrical energy by reacting a fuel with an oxidant, usually in the presence of a catalyst. Typically, fuel cells consist of a fuel electrode, or anode, and a reducing electrode, or cathode, separated by an ion-conducting electrolyte. An external circuit conductor connects the electrodes to an electrical circuit, or load. In the conductor, current is transported by the flow of electrons. In the electrolyte, current is transported by the flow of ions.
Any number of hydrogen rich fuels may be used as a fuel source, such as methanol, ethanol, butane, and propane. FIG. 1 is a diagram of a methanol fuel cell. A reservoir that includes the anode, or anode reservoir 102, contains a methanol-water solution 104. The methanol fuel cell generally is in a charged state when the percentage of methanol in the methanol-water solution is relatively large. As methanol is oxidized and electricity is generated by the fuel cell, the percentage of methanol in the methanol-water solution decreases and the fuel cell becomes depleted.
The methanol contained within the methanol-water solution is oxidized, usually in the presence of a catalyst, producing hydrogen ions 106, electrons 108, and carbon dioxide 116. This oxidation reaction occurs inside the anode reservoir 102 of the fuel cell. A primary anode oxidation reaction is shown below:CH3OH+H2O→CO2+6H++6e−Note that, since the electrolyte is a relatively poor electrical conductor, electrons 108 flow away from the anode via an external circuit 110. Simultaneously, hydrogen ions 106 travel through the electrolyte, or membrane 112, to the cathode 114. Commonly used membranes include Nafion 112®, Nafion 117®, and polybenzimidazole.
At the cathode 114 of a fuel cell, oxygen 118 is reduced by hydrogen ions 106 migrating through the electrolyte 112 and incoming electrons 108 from the external circuit 110 to produce water 120. The primary cathode reaction is shown below:3/2O2+6H++6e−→3H2OThe individual electrode reactions, described above as primary anode and primary cathode reactions, result in an overall methanol-fuel-cell reaction shown below:2CH3OH+3O2→2CO2+4H2O+electricityAdditional minor chemical reactions may occur, and thermal energy is generally produced.
Modern fuel cells can continuously produce electrical current for long periods of time without the need for recharging. However, fuel cells produce electrical charge only when fuel is present in the anode reservoir above a threshold concentration. Therefore, in order to ensure continuous operation of a fuel cell, an indication of the amount of fuel remaining in the fuel cell needs to be easily obtainable. Fuel cells commonly provide no convenient, cost-efficient means for reliably determining the amount of available fuel remaining in the fuel cell. Therefore, designers, manufacturers, and users of fuel cells have recognized the need for a convenient, cost-efficient means for determining the amount of fuel remaining in a fuel cell.